In general, radiotherapy treatment systems involve a source of ionising radiation, such as a linear accelerator, and a patient support system (PSS) on which the patient can be placed, in the field of view of the source.
The source is rotateable around the PSS so that a radiation dose can be applied to the patient from a number of directions in order to minimise the dose applied to healthy tissue, and the intersection of the axis of rotation with the beam centreline defines the “isocentre”. The PSS is moveable so as to assist in aligning the patient with the isocentre; in general a clinician will wish to place the tumour at the isocentre, although this may not always be the case. Given that the source and the PSS are both moveable, it will be necessary to ensure that they are both aligned to the same frame of reference so that the clinician can be confident that the patient is indeed in the correct position.
A modern PSS is moveable in up to six degrees of freedom with a high degree of accuracy, these being translation in three directions, rotation about a vertical axis, and tilting in two directions. To calibrate this, a reference frame can be affixed to the PSS surface, which has reflective spheres in a known and asymmetric arrangement. These are viewed by an infra-red camera and the image is analysed. Given that the arrangement of the spheres is known a priori, the orientation of the reference frame and hence the table can be determined.
The source commonly includes both a therapeutic beam and a correlated diagnostic beam. A flat panel imager is placed opposite the diagnostic beam to obtain an image, and this can be used during the treatment process to image the tumour and update its position. During calibration, the return from the diagnostic imager can be used to identify the isocentre location.
It therefore remains only to correlate the two sets of location data. To do so, laser calibration beams directed at the isocentre have hitherto been used. These illuminate the isocentre and therefore make it visible, allowing the reference frame to be aligned with the isocentre by moving the PSS, after which the frame location is determined via the camera system. Thus, the PSS frame of reference is calibrated relative to the isocentre.